"GO now!" The message came from Pakistan and it rattled the British intelligence agents who intercepted it.

The urgent direction was sent early this week to a group of young British Muslims who had been plotting one of the world's biggest-ever terrorist attacks.
Intelligence officials who had been monitoring the group thought they had plenty of time to keep watching them and gathering evidence before safely rounding them up early enough to stop them carrying out their scheme.

But the message from radicals in Pakistan saying "do the attacks now" changed everything, and when it was passed to higher authorities in London on Wednesday night, it sent a jolt through the top levels of the British Government.

Prime Minister Tony Blair had jetted off on Monday for a holiday at pop star Cliff Richard's mansion in Barbados, believing that the long-running investigation of the terror plot was some time away from a climax.

As late as 8pm London time on Wednesday, when Mr Blair called US President George W Bush for an arranged discussion, he mentioned the investigation but still did not know that it had suddenly become urgent.

His Transport Minister, Douglas Alexander, who is responsible for British aviation, was on holiday in Scotland and even Andy Hayman, the assistant commissioner of police in charge of specialist operations, was enjoying the sun in Spain.

Commissioner Hayman's deputy, Peter Clarke, was told late on Wednesday about the hurry-along message from Pakistan, which may have been prompted by the arrest of a co-conspirator in Pakistan, and decided the British suspects had to be arrested straight away.

By dawn on Thursday, all 24 known suspects had been grabbed in raids on houses in London, Birmingham and High Wycombe, and a mosque in Birmingham.

They are mostly in their late teens and early 20s, the youngest 17 and the oldest 35. All are British citizens.

Many are well-educated and come from middle-class families who own businesses and property.

'First whiff'

There are various accounts of how the plot first came to the attention of British authorities.

The Washington Post quoted US officials as saying "the first whiff" had come after last year's July 7 suicide bombings in London, when a Muslim tipped off British authorities to the suspicious activities of an acquaintance.

That first tip was vague but it led investigators to a well co-ordinated plot to bomb transatlantic flights, the paper said.

The Times, on the other hand, claimed that MI5 first noticed some of the men last year because of their suspicious behaviour during frequent trips to Pakistan, while the Daily Mirror quoted an intelligence source as saying that the agency had been alerted while monitoring visitors to extremist websites which displayed information on making bombs.

Phillip Knightley, the London-based Australian journalist and author, who specialises in security issues, said the initial information about the plot had probably come from an informer, with intelligence officials intentionally clouding the issue to protect their source.

MI5, which has enjoyed a big funding boost since 2001, has devoted most of its new resources to monitoring British Muslims suspected of having al Qaeda sympathies.

Like the July 7 bombers, many of those radicals have been motivated by anger at British foreign policy in Iraq, Afghanistan and other Muslim countries, and MI5 has worked hard to monitor formal and informal groups opposed to British military activities, and young British Muslims who travel to Pakistan.

The intelligence service called in anti-terrorism police and put 50 people under surveillance before focusing on two dozen suspects.

Investigators eventually fitted 12 vehicles with GPS tracking devices, monitored calls from mobiles and pay telephones and tapped into emails sent to Pakistan, Europe and Iran.

They followed their targets, recorded their meetings and conversations, secretly scrutinised their bank accounts, and took note of what they read on the internet, where they shopped and how they spent their money.

"We have been looking at meetings, movements, travel spending and the aspirations of a large group of people," said Peter Clarke, the head of Scotland Yard's anti-terrorism branch.

By March, they realised a major attack was being planned and by April, the Prime Minister had been informed.

CNN has quoted "several US government officials" as saying that an undercover British agent had infiltrated the group.

In any case, the plot became the biggest anti-terror surveillance operation in British history, with more than 1500 people working on it.

When the suspects inquired about the prices of flights to the US and a dozen of them went to the American embassy in London to apply for tourist visas, the FBI was informed.

Pakistan's military intelligence service also provided valuable information, and MI6, which deals with foreign security threats, consulted counter-intelligence agencies in Europe and northern Africa. US officials claimed yesterday that large sums of money were sent to leaders of the group from Pakistan to allow them to buy airline tickets.

British officials said several of the men had large sums of money in their bank accounts that could not be explained by their normal incomes.

Intelligence agents were adamant that the plotters had been within two days of conducting a test run to see if they could carry liquid explosives and detonating devices on to commercial fights to the US, and if they had succeeded, they would have quickly gone ahead with the mission itself.

There were conflicting reports on whether the plan was to destroy five, 10 or 12 jets, and whether they were to be brought down over US cities to maximise casualties or over the sea so that the method of explosion could not be detected.

What is agreed by British and US officials is that the suspects had already studied timetables of three US airlines, Continental, United and American, and if their plot had succeeded, its death toll could easily have exceeded the 2976 who died on September 11, 2001.